Even though the present invention is described below in connection with radar systems for vehicles, it is not limited to this and may be used with any radar system.
In modern vehicles, a large number of electronic systems is used, for example, if they are able to support the driver in his guidance of the vehicle. Braking assistants, for instance, are able to detect preceding traffic participants and to brake and accelerate the vehicle accordingly, so that a specified minimum distance is always maintained from the preceding traffic participants. Such braking assistants are also able to initiate emergency braking, when they detect that the distance from the preceding traffic participant is becoming too small.
In order to be able to provide such assistance systems in a vehicle, it is necessary to record data about the surroundings of the respective vehicle. In the above example of a braking assistant, it is required, for instance, to record the position of a preceding traffic participant, in order to be able to calculate the distance of one's vehicle from the preceding traffic participant.
In the detection of the position of a preceding traffic participant, the azimuth angle, for example, of the preceding traffic participant is able to be recorded, starting from the driving direction of the respective vehicle. Since functionally non-relevant objects such as manhole covers or bridges also reflect radar signals, the detection of the angle of elevation permits one to distinguish between functionally relevant and non-relevant objects.
Such a detection of the azimuth angle or angle of elevation may take place, for example, by evaluation of the phases and amplitudes of the receiving antennas of a radar system.
US document US 2012/256795 A1 shows a possible antenna for such a radar system.
For a two-dimensional antenna array having phase centers xi and yi, the following equation applies for the phase on element i:
      φ    i    =                    2        ⁢        π            λ        ⁢          (                                    x            i                    *          sin          ⁢                                          ⁢          θ          *          cos          ⁢                                          ⁢          Φ                +                              y            i                    *          sin          ⁢                                          ⁢          Φ                    )      where θ represents the azimuth angle and Φ the angle of elevation.
In a general two-dimensional antenna array, the azimuth angle and the angle of elevation have to be calculated jointly. Because of that, the calculating expenditure rises sharply. It is therefore desirable to decouple the calculation of the azimuth angle and the angle of elevation.
It is known to the Applicant that one should use an antenna as shown in FIG. 8, in order to enable a separate calculation of the azimuth angle and the angle of elevation for small angles of elevation (cos(Φ)≈1). FIG. 8 shows the positions of the receiving elements of an antenna. In this context, the four receiving elements for the calculation of the azimuth angle are situated in a horizontal plane. The two additional receiving elements for the calculation of the angle of elevation are situated in a vertical plane above the third receiving element of the horizontal plane.
To be sure, in modern radar systems for vehicles, a higher antenna gain is required in the elevation direction, in order to achieve focusing on the relevant elevation angle range, and thus to be able to fade out interferences such as road clutter.
In order to make possible a high antenna gain, the antenna elements of such an antenna need a large extension into the direction of elevation. In the case of the system of FIG. 8, this leads to a very large extension in the elevation direction. However, the greater the extension into the elevation direction, the less is the unambiguity range in the elevation angle estimation. In this context, the unambiguity range is a function of the frequency of the radar signals used. This comes about from the evaluation of the phase position of the individual receiving elements, in which the phase shift is able to amount at the most from −π to +π. If the phase shift is greater, the signal can no longer be associated unambiguously. As a result, a greater distance of the individual receiving elements leads to a reduced unambiguity range, since the phase position shifts because of the greater distance.